I tend to agree with most of what you're saying i think. I see the modern divisions between Ireland and Scotland as somewhat misleading for this period. I wouldn't say Dal Riata was 'irish' as such as i think that puts a modern spin on things, but i agree that culturally these areas were similar. Argyll and the southern hebrides and much of Ulster can really be seen as a single unit. I go with the idea that the supposed 'invasion' of west scotland by the Scots was nothing of the sort. This was predominantly later attempts to explain why the west coast of Scotland had a Gaelic culture. Rather what existed was a cultural and geographical unit encompassing the north of Ireland and Argyll and some other parts of Scotland. Of course this was a dynamic process and power structures shifted between various areas within this unit at different times and interactions with external bodies had strong effects, but the so-called invasion is unlikely to have ever happened. The connections and existence of this socio-cultural sphere had always been there.